TWI425757B - Power converter and related method - Google Patents

Description

Power converter and related method

The present invention relates to a power converter, and more particularly to a power converter and associated method for adjusting the overcurrent protection mechanism with an output of an auxiliary winding.

Please refer to FIG. 1. FIG. 1 is a schematic circuit diagram of a flyback AC/DC power converter 100 for receiving an AC line voltage V _AC to provide an output voltage V _out to the load element 101. The signal obtained by the half-wave voltage corrects the overcurrent protection level, so that the overcurrent protection mechanism can be triggered early when the AC line voltage is high. As shown, the power converter 100 includes a bridge rectifier 105, a transformer 110, rectifier diodes D _out and D _aux , a switching element 115, a control circuit 120, a detection circuit 125, and a compensation resistor R _comp . System control circuit 120 controls the duty cycle of the switching element 115 to control the magnitude of the output voltage V _out, and by the detection circuit 125 to know whether the current flowing through the switching element 115 of the transformer 110 exceeds one current limit value I _limit ( That is, an overcurrent occurs). If an overcurrent occurs, the control circuit 125 turns off the switching element 115 to avoid causing damage to the inductance in the switching element 115 or the transformer 110.

Voltage source the voltage V _in the allowable range, in the event of an overcurrent, the power converter 100 should be designed to output the output power source voltage V _out is a fixed value. When Voltage source V _in voltage to a high voltage (high line), instance 264 Volts, when the overcurrent occurs, the power converter 100 may operate in a discontinuous conduction mode (discontinuous conduction mode), which is converted in a switching cycle The energy can be derived as P _t - _-265 = 1/2 × L _P × I _{limit - 265V} ² , where L _P is the inductance value of the main winding of the transformer 110, and I _{limit - 265V} is the voltage source V _in voltage Current limit value. However, when the voltage source V _in voltage is a low voltage (low line), instance 90 volts, when an overcurrent occurs, the power converter 100 may operate in continuous conduction mode (continuous conduction mode), which is in a switching cycle The converted energy can be derived as P _t-90 = 1/2 × L _P × (I _{limit - 90V} ^{2 -} I _{O - 90V} ² ), where I _{limit - 90V} is the current limit value of the voltage source V _in voltage, I _O-90V is the initial value of the current of the main winding of the transformer 110. It can be found that if P _t-265 = P _t-90 , I _limit-90V must be designed to be larger than I _limit-265V . The compensation resistor R _comp is used to reduce the current limit value I _limit as the voltage source V _in voltage increases.

The compensation resistor _Rcomp is used to boost the voltage level of the forward input of the comparator 1250 in accordance with the voltage after the bridge rectifier 105, thereby correcting the current limit value _{Ilimit of} the overcurrent protection. It can be seen from the circuit that when the voltage source V _in is high line, the compensation resistor R _comp provides a certain high voltage level V _{lift at} the forward input end of the comparator 1250, so the current flowing through the switching element 115 Only a small amount is needed, so that the voltage level of the positive input terminal of the comparator 1250 reaches the voltage level _Vth , and the overcurrent protection is triggered. Therefore, the current limit value I _limit detection circuit 125 will V _in voltage as the voltage source rises.

However, the design of the compensation resistor R _comp has the disadvantage of reducing the energy saving capability of the power converter 100 (described below):

1. Compensation is provided a resistance R _comp by the voltage source V _in to a fixed ground line leakage paths, wasteful power consumption is fixedly.

2. Power converter 100 may not be able to enter power save mode during high line light load or no load. The power saving mode needs to be triggered by the output voltage source V _out output voltage reverse one of the compensation signals V _COM below a certain level V _COM-BURST . There is a voltage control mode called a current output mode to output the output voltage V _out of the voltage source to limit the current flowing through the switching element 115 of the peak. In practice, the voltage level at the positive input terminal of the comparator 1250 is compared with the compensation signal V _COM , and the comparison result controls the switching element 115. In the circuit of Figure 1, the compensation resistor _Rcomp fixedly provides a certain voltage level _{Vlift at} the forward input of the comparator 1250, regardless of light load or heavy load. If the compensation signal V _{COM is} lower than a corresponding voltage level V _lift , the switching element 115 will continue to be turned off, the power converter 100 will not convert energy, and the output voltage of the output voltage source V _out will not rise again, and the compensation The signal V _COM will not be lowered. Therefore, the voltage level V _lift corresponds to the lowest point V _{COM-MIN of} the compensation signal V _COM . At high voltage, the voltage level V _{lift is} relatively high, and the lowest point of the compensation signal V _COM-MIN is also relatively high; in case the compensation signal lowest point V _{COM-MIN is} higher than the V _COM- triggered by the power saving mode. _BURST , that means the power saving mode will not be triggered at all.

According to an embodiment of the invention, a power converter is disclosed. The power converter includes a transformer, a switching component, a control circuit, an overcurrent detecting circuit and a compensation signal generating circuit, wherein the transformer has a primary winding and at least one auxiliary winding. The control circuit is coupled to a control end of the switching element and configured to control the switching element to be turned on or off to control a primary current flowing through the main winding, and the overcurrent detecting circuit is And comparing the main current and a current limit value, when the main current is higher than the current limit value, the over current detecting circuit causes the control circuit to turn off the switching element, and the compensation signal generating circuit is coupled to the Between the auxiliary winding and the overcurrent detecting circuit, and when the switching element is turned on, outputting one of the auxiliary windings to provide an adjustment value to adjust the current limit value; wherein, when the switching element is turned on, This adjustment value will change over time.

According to an embodiment of the present invention, there is further disclosed a method for a power converter, wherein the power converter includes at least one switching element and a transformer having a main winding and at least one auxiliary winding, and the method includes the following Step: controlling the switching element to be turned on or off to control a main current flowing through the main winding; comparing the main current with a current limit value, when the main current is higher than the current limit value, turning off the switching element; And when the switching element is turned on, one of the auxiliary windings is output, and one of the adjustment values is changed with time to adjust the current limit value.

Referring to FIG. 2, FIG. 2 is a circuit diagram of the flyback power converter 200 according to the first embodiment of the present invention. The power converter 200 receives an AC line voltage V _AC to provide an output voltage V _out to the load element 201, and includes a bridge rectifier 205, a transformer 210, rectifying diodes D _out and D _aux , and capacitors C _out and C _{The aux} , the switching element 215, the control circuit 220, the overcurrent detecting circuit 225, and the compensation signal generating circuit 230. The transformer 210 includes a primary winding W ₁ , a secondary winding W ₂ , and windings W ₃ and W ₄ . The windings W ₃ and W _{4 are} connected in series to form an auxiliary winding. Winding W ₃ of an output line for providing operating power to the control circuit 220 V _cc. The output of winding W ₄ is coupled to compensation signal generating circuit 230. The switching element 125 to a transistor-based implementation of, for controlling the flow through the main winding W ₁ of the main current.

In this embodiment, the control circuit 220 is a pulse width modulation (PWM) control circuit, which outputs a control signal V _c according to the output voltage V _out to control the switching element 215 to be turned on or off. (on / off) to control the duty cycle of the switching element 215 (duty cycle), thus achieving the object W ₁ and the amount of current of the main switching element 215 controls the magnitude of output voltage V _out and flowing through the main winding. It is detected by the overcurrent detecting circuit 225 to detect whether an overcurrent has occurred. The overcurrent detection circuit 225 compares the main current with a current limit value _Ilimit . When the main current is higher than the current limit value _Ilimit , the overcurrent detection circuit 225 notifies the control circuit 220 to turn off the switching element 215. In detail, the overcurrent detecting circuit 225 includes resistors R _s and R _s ' and a comparator 2250. The voltage level on the connection node N ₁ between the switching element 215 and the resistor R _s (also referred to as a detecting resistor) It can roughly represent the amount of current flowing through the switching element 215 (i.e., the amount of the above main current). Therefore, the operating principle of the comparator 2250 is to compare the voltage level on the connection node N ₁ with a threshold voltage V _th . If the voltage level on the connection node N ₁ is higher than the threshold voltage V _th , it means that the occurrence has occurred. The current, the comparator 2250 outputs a signal notification control circuit 220, and the control circuit 220 turns off the switching element 215 according to the output of the comparator 2250. Assuming that the compensation signal generating circuit 230 does not provide any adjustment value, the compensation signal generating circuit 230 is an open circuit. The current limit value I _limit is approximately constant, which is approximately equal to the threshold voltage V _th divided by the resistance R _s . resistance.

The compensation signal generating circuit 230 is configured to provide an adjustment value to adjust or compensate the voltage level of the forward output terminal of the comparator 2250, and equivalently adjust the current limit value I _limit . For example, when the compensation signal generating circuit 230 pulls the voltage level of the forward output terminal of the comparator 2250 high, the current flowing through the main winding W ₁ and the resistor R _s can be compared under the condition that the comparator 2250 is The voltage at the forward output reaches the threshold voltage _Vth , triggering overcurrent protection. Therefore, the current limit value I _limit used to detect the overcurrent is equal to being reduced.

In operation, the compensation signal generating circuit 230 provides an adjustment value to adjust the current limit value I _limit with the output of the winding W ₄ when the switching element 215 is turned on. Winding the auxiliary winding W ₃ provides operating power _{to the} V _cc when the switching element 215 to control circuit 220 off. In the present embodiment, the compensation signal generating circuit 230 includes a rectifying diode D ₁ , resistors R ₁ and R ₂ , a Zener diode D _{z ,} and a capacitor C ₁ . Resistor R ₁ and capacitor C ₁ can be considered as a low pass filter. Referring to FIG 3, which is depicted in FIG. 2 as shown in the waveform of the control signal V _C, the voltage waveform V across winding W ₄ of the transformer _2104, the rectifying diode D ₁ output voltage waveform V ₄ ', and the voltage across the capacitor C ₁ V _C1 . As shown in Fig. 3, the voltage waveform V ₄ has a periodic change, and the change at time T ₁ , T ₃ is due to the non-conduction of the switching element 215, and changes at times T ₂ and T ₄ . due to the transistor switching element 215 is turned on, the voltage across the inductor main winding W ₁ is generated. The rectifying diode D ₁ filters out signal components below its own threshold voltage V _D1 . When the voltage value of the voltage waveform V ₄ ' is higher than the reverse breakdown voltage of the Zener diode D _z , the low-pass filter starts to operate, and the voltage across the V _C1 begins to rise with time as shown. The voltage across the voltage V _C1 will affect the voltage at the forward output of the comparator 2250 through the resistor R ₂ , which is equivalent to changing the current limit value I _limit used by the overcurrent detection circuit 225 for comparison.

The compensation signal generating circuit 230 can be designed to generate almost no adjustment value to adjust or compensate the voltage of the forward output terminal of the comparator 2250 when the power converter 200 is under light load or no load. At light load or no load, the duty cycle will be relatively small, that is, the on-time of the switching element 215 will become relatively short. Referring to Fig. 3, when time T ₂ and T ₄ are short, waveform V ₄ ' cannot effectively charge capacitor C ₁ , and its charged crossover voltage V _C1 will be relatively small and can be regarded as zero. . In other words, the low-pass filter formed by the resistor R ₁ and the capacitor C ₁ filters out the influence of the small duty cycle, and equivalently does not provide an adjustment value to correct the voltage at the forward output of the comparator 2250. The current limit value I _limit used by the overcurrent detection circuit 225 for comparison is adjusted. On the other hand, when the power converter 200 is in a heavy load, the duty cycle becomes large, and the signal width of the waveform V ₄ ' (ie, the times T ₂ , T ₄ ) also becomes longer, and after charging The voltage across the capacitor C ₁ , V _{C1 ,} will become large and cannot be ignored, so that the current limit value I _limit used by the overcurrent detection circuit 225 for comparison can be significantly adjusted.

In addition, when the power converter 200 is in the low line of the AC line voltage V _AC , the compensation signal generating circuit 230 can be designed to generate almost no adjustment value adjustment or compensate the voltage of the forward output terminal of the comparator 2250; When the AC line voltage V _AC is high line, it will be. A third voltage waveform of the voltage value V of the A _{1 4} V _in the induction voltage is generated, and after the rectified voltage V _in is the AC line voltage V _AC with a positive correlation. It can be seen from FIG. 3 and FIG. 2 that the voltage value A ₁ must be high enough to overcome the threshold voltage of the diode D1 and the reverse breakdown voltage of the Zener diode D _z to charge the capacitor C ₁ . It is possible to influence the current limit value I _limit used by the overcurrent detecting circuit 225 for comparison. In short, the output current V _{4 of the} winding W ₄ must be higher than a predetermined voltage value to adjust the value of the overcurrent detecting circuit 225. When the threshold voltage of the diode D1 is ignored, the voltage preset value is roughly determined by the reverse breakdown voltage of the Zener diode. Therefore, the voltage of the AC line voltage V _AC needs to be high to a certain extent, which may affect the overcurrent detecting circuit 225. For example, if the appropriate Zener diode is selected, the compensation signal generating circuit 230 can be used. When the power converter 200 is at an AC line voltage V _AC lower than 180 volts, the adjustment value adjustment or compensation comparator 2250 is not generated. The voltage at the forward output; however, it is only possible to adjust or compensate for the voltage at the forward output of comparator 2250 when the AC line voltage V _{AC is} above 180 volts.

In an embodiment, by appropriately selecting the component values in FIG. 2, the compensation signal generating circuit 230 can affect the overcurrent detecting circuit 225 when the high voltage is reloaded, and adjust or compensate the positive of the comparator 2250. The voltage to the output. At low voltage or light load, there is no problem of transmitting signal delay, or there is no problem of overcurrent, and the compensation signal generating circuit 230 does not affect the current detecting circuit 225.

The embodiment in Fig. 2 does not have the fixed leakage path in Fig. 1, so that as long as the switching element 215 is turned off, the second picture has almost no power consumption and is very energy-saving. The embodiment in Fig. 2 also eliminates the problem that the power saving mode cannot be entered when the high line light load or no load is used in Fig. 1, because the embodiment in Fig. 2 It can be set to have no effect on the current detecting circuit 225 when light load or no load is applied.

In an embodiment, the compensation signal generating circuit 230 can also be implemented in the form of a circuit as shown in FIG. 4A. In an embodiment, the Zener diode D _z in the compensation signal generating circuit 230 shown in FIG. 4A can also be disposed between the rectifying diode D ₁ and the resistor R ₁ ; The design changes made to the position of the circuit elements under the operation of the compensation signal generation circuit 230 to provide the adjustment value are all within the scope of the present invention.

The Zener diode D _{z is} not limited to operate in conjunction with the low pass filter formed by the resistor R ₁ and the capacitor C ₁ . Therefore, in other embodiments, the compensation signal generating circuit 230 can utilize the 4th or the The circuit form of the 4C diagram is implemented, and this still has the function of providing an adjustment value to adjust the current limit value I _limit , and retains the energy saving effect.

Transformers can also have different designs. Referring to FIG. 5, FIG. 5 is a circuit diagram of a flyback power converter 500 according to a second embodiment of the present invention. Different from the transformer 210 of the power converter 200, the transformer 510 included in the power converter 500 has only one winding W ₅ as an auxiliary winding, and the connection of the rectifying diode D _aux is also different (such as the fifth As shown, the auxiliary winding W ₅ is used to provide its output to the compensation signal generating circuit 230 to provide the adjustment value when the switching element 215 is turned on, and to provide the operating power signal V _cc when the switching element 215 is turned off. Control circuit 220. Of course, the circuit design of the compensation signal generating circuit 230 shown in FIG. 5 can also be implemented by using the circuit form shown in FIGS. 4A-4B, which is also in accordance with the spirit of the present invention.

The above are only the preferred embodiments of the present invention, and all changes and modifications made to the scope of the present invention should be within the scope of the present invention.

100, 200, 500. . . Flyback power converter

101, 201. . . Load element

105, 205. . . Bridge rectifier

110, 210, 510. . . transformer

115, 215. . . Switching element

120, 220. . . Control circuit

125. . . Detection circuit

225. . . Overcurrent detection circuit

230. . . Compensation signal generation circuit

1250, 2250. . . Comparators

Figure 1 is a circuit diagram of a conventional flyback power converter.

2 is a circuit diagram of a flyback power converter according to a first embodiment of the present invention.

A waveform control signal V _c shown in the third graph of FIG. 2, the second winding of the transformer on the output voltage waveform W ₄ V _4, a rectifying diode D ₁ of the output voltage waveform V ₄ 'and a capacitor C ₁ Schematic diagram of the cross-pressure V _C1 .

Fig. 4A is a circuit diagram showing another variation of the implementation of the compensation signal generating circuit shown in Fig. 2.

Fig. 4B is a circuit diagram showing a third implementation variation of the compensation signal generating circuit shown in Fig. 2.

Fig. 4C is a circuit diagram showing a fourth implementation variation of the compensation signal generating circuit shown in Fig. 2.

Fig. 5 is a circuit diagram showing a flyback power converter according to a second embodiment of the present invention.

200. . . Flyback power converter

201. . . Load element

205. . . Bridge rectifier

210. . . transformer

215. . . Switching element

220. . . Control circuit

225. . . Overcurrent detection circuit

230. . . Compensation signal generation circuit

2250. . . Comparators

Claims (12)

A power converter includes: a transformer having a primary winding, a secondary winding, and at least one auxiliary winding, wherein the secondary winding is used Generating an output voltage and providing the output voltage to a load component; a switching component; a control circuit coupled to the control terminal of the switching component for controlling the switching component to be turned on or off to control flow through the a primary current of the main winding; an overcurrent detecting circuit for comparing the main current with a current limit value, and when the main current is higher than the current limit value, the overcurrent detecting circuit makes the current The control circuit turns off the switching element; and a compensation signal generating circuit is coupled between the auxiliary winding and the overcurrent detecting circuit, and when the switching element is turned on, the output of the auxiliary winding is not referenced to the secondary winding Any of the output signals provides an adjustment value to adjust the current limit value; wherein the adjustment value changes with time when the switching element is turned on. The power converter of claim 1, wherein the compensation signal generating circuit has a low pass filter coupled between the auxiliary winding and the overcurrent detecting circuit. The power converter of claim 2, wherein the low pass filter comprises: a resistor connected in series between the auxiliary winding and the overcurrent detecting circuit; and a capacitor having a first The end is coupled to the resistor and coupled to a second terminal to a reference level. The power converter of claim 1, wherein the compensation signal generating circuit adjusts the current limit value when an output voltage of the auxiliary winding is higher than a voltage preset value. The power converter of claim 4, wherein the compensation signal generating circuit further comprises: a Zener diode coupled between the auxiliary winding and the overcurrent detecting circuit, Used to determine the voltage preset value. The power converter of claim 1, wherein the overcurrent detecting circuit comprises: a detecting resistor connected in series with the switching element through a connecting end; and a comparator coupled to the The main current and the current limit value are compared between the connection end and the control circuit by detecting the voltage across the detection resistor. The power converter of claim 1, wherein the auxiliary winding is When the switching element is turned off, an operating power is supplied to the control circuit; when the switching element is turned on, the current limit value can be adjusted. The power converter of claim 7, wherein the auxiliary winding comprises: a first winding and a second winding connected in series; wherein an output of the first winding is used to provide the operation The power is supplied to the control circuit, and an output end of the second winding is connected to the compensation signal generating circuit. A method for a power converter, wherein the power converter includes at least one switching element and a transformer having a main winding, a primary winding, and at least one auxiliary winding, the secondary winding is configured to generate an output voltage, And providing the output voltage to a load component, and the method includes: controlling the switching element to be turned on or off to control a main current flowing through the main winding; comparing the main current with a current limit value when the main When the current is higher than the current limit value, the switching element is turned off; and when the switching element is turned on, one of the auxiliary winding outputs without reference to any signal output by the secondary winding, providing one of changes with time Adjust the value to adjust the current limit value. The method of claim 9, wherein the step of adjusting the current limit value is performed when an output voltage of one of the auxiliary windings is higher than a voltage preset value. The method of claim 9, wherein the step of comparing the main current with the current limit value comprises: connecting a detecting resistor and the switching element in series; and detecting a cross voltage of the detecting resistor The main current is compared to the current limit value. The method of claim 9, further comprising: providing an operating power to the control circuit via the auxiliary winding when the switching element is turned off.